Rt gear unphysical check fix

For now, catch either mass or rho being zero. At the moment, they are not necessarily both zero. For example, an unphysical check may zero out both mass and density when it becomes negative in a hydro step. Once that happens, particles may gain mass through flux exchanges with other active particles, while they themselves remain inactive. The density of such inactive particles however remains zero until the particle is active again. See issue #833.